This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Our goal for our efforts at the RLBL is to complete a proof-of-principle experiment based on a theoretical model we developed to characterize nanoscale heterogeneity (domains) within fluid bilayers. Briefly, the experiments consist of time-correlated single photon counting (TCSPC) experiments on a suspension of small unilamellar vesicles. These vesicles, composed of various ratios of DMPC/cholesterol, are proposed to be in a two-phase coexistence regime. However, to date, other techniques such as fluorescence microscopy indicate that if there is coexistence, it must be on a scale below the optical limit. Our theory utilizes time-resolved data in these systems that contain two fluorophore-labeled lipid probes undergoing resonance energy transfer within the bilayer geometry. Fitting the data with our model should yield information about the characteristic size of any domains within the bilayer.